The present invention relates to a method and apparatus therefor, for an improved laser-cooling fluorescence spectrometry as one type of ion trapping mass spectrometry.
Ion trapping mass spectrometry is a widely used method for trace analysis in environmental analysis and other fields. In the most typically utilized method, ions trapped in an ion trap are subjected to mass selection and extracted outside the trap, and these extracted ions are detected with an ion detector such as an electron multiplier. This type of mass spectrometry is still the most widely used method and a large number of reference works are available (such as Reference 1: R. E. March and R. J. Hughes: xe2x80x9cQuadrupole Storage Mass Spectrometryxe2x80x9d John Wiley and Sons (1989)).
Laser-cooled fluorescence mass spectrometry relating to this invention is found in a method disclosed in 1995 (Reference 2: U.S. Pat. No. 5,679,950) as a novel ion trap mass spectrometry. In this method, laser-cooling and sympathetic cooling are utilized and sensitivity which has been limited to detection levels of about 100 ions in the ion trapping mass spectrometry of the prior art, is significantly improved by optical detection of the cooled sample ions so that even single ions can be detected in-situ. In this method, the sample ions are trapped in the ion trap after mass analysis and can be measured repeatedly to detect ions in an xe2x80x9cin-situxe2x80x9d (or non-destructive) manner.
The principle of laser-cooled fluorescence mass spectrometry is briefly described below.
Ions trapped in a radio-frequency-quadrupole ion trap possess a harmonic oscillation mode due to the potential of the ion trap (in pseudo-potential approximation). The oscillation of these trapped ions is known as secular (oscillation) motion. The frequencies of these secular oscillations are proportional to the charge of the ions and are inversely proportional to the mass. If the secular frequency can be detected, then the mass spectrometry of the ions can be performed (Reference 1).
Firstly, the laser-cooled ions and sample ions are simultaneously trapped and cooled. The sample ions are sympathetically cooled by the laser-cooled ions. Next, an electrical oscillation is applied to make the sample ions resonate at the secular frequency, so that the sample ions are heated by the resonant oscillation. The sample ions are repeatedly undergoing Coulomb collision with the laser-cooled ions so that the sample ions transfer energy to the laser-cooled ions and the laser-cooled ions are sympathetically heated.
Increase of the temperature of the laser-cooled ions results in such effects as change of the fluorescence intensity, and change in the spatial distribution of the fluorescence. These changes give information on the mass and the amount of the sample ions.
The mechanism of fluorescence intensity change in the laser-cooled fluorescence mass spectrometry can be understood by the following brief theoretical analysis.
First, using a simplified theory of Doppler laser-cooling, the relation of laser cooling efficiency and fluorescence intensity is calculated with respect to ion temperature.
When a laser beam with a fixed wave vector irradiates free atoms (or ions), a force acts on the atoms in the direction of the wave vector of the light due to photon scattering. Laser cooling is performed using this force. Typically, an atom having a simple two-level transition is chosen to avoid optical pumping, and the laser wavelength is adjusted to the resonance transition of the two-level atom species.
When the momentum of the atom is counter to the direction of the wave vector of the laser light, the velocity of the atoms decreases after resonant absorption of photons due to momentum transfer of photons to the atom. Conversely, when the momentum of the atom is in the same direction of the wave vector of the laser light, then the velocity increases.
When the detuning frequency is negative, in other words, when the light has a wavelength slightly longer than the resonant wavelength, the probability of resonance scattering increases when the atoms are traveling counter to the direction of laser light wave vector, compared to when traveling in the same direction to the light wave vector due to the Doppler effect. In this case, consequently, the energy of the atom is lost and the atom cools down.
Spatially-uniform natural emission following absorption results in a random-walk increase of energy, whose balance with the cooling effect determines the ultimate attainable temperature h xcex93/2, where xcex93 is the natural linewidth of the transition. As shown below, the average energy change due to resonant absorption is h xcex94xcexd, where xcex94xcexd is the detuning frequency, which is the deviation of the laser light frequency from the resonant frequency xcexd0 of the atoms at rest. When |h xcex94xcexd| is greater than h xcex93/2, we can neglect the random-walk heating by natural emission, which is the case considered below to simplify discussion.
An average energy change h xcex94xcexd due to resonance absorption of photons can be explained by simple kinetics. The wave vector direction of the laser light is set as the z axis. The velocity component along the z axis of the atom with mass m is defined as vz in the laboratory frame. In the center of mass system, the frequency of the light shifts xcexd(1xe2x88x92vz/C) due to the Doppler effect. When xcexd(1xe2x88x92vz/C) matches the resonant frequency xcexd0, a resonant scattering occurs. The atom obtains momentum h xcexd0/c from the laser light when the atom absorbs the light. Next, the atom emits photon by spontaneous emission process. This photon emission is uniform in all directions so net average change of the atom momentum does not occur. Consequently, the atom obtains momentum h xcexd0/C by the resonant scattering of the laser light.
The velocity after the resonant scattering becomes vzxe2x80x2=vz+h xcexd0/mc when observed in the laboratory frame. The change xcex94E of kinetic energy is                                                                         Δ                ⁢                                  xe2x80x83                                ⁢                E                            =                              xe2x80x83                            ⁢                                                                    mv                    z                                          xe2x80x2                      ⁢                                              xe2x80x83                                            ⁢                      2                                                        2                                -                                                      mv                    z                    2                                    2                                                                                                        =                              xe2x80x83                            ⁢                              h                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                                  ν                  ⁡                                      (                                          1                      +                                                                        h                          ⁢                                                      xe2x80x83                                                    ⁢                          Δ                          ⁢                                                      xe2x80x83                                                    ⁢                          ν                                                                          4                          ⁢                          E                                                                                      )                                                                                                                          ≅                              xe2x80x83                            ⁢                              h                ⁢                                  xe2x80x83                                ⁢                Δ                ⁢                                  xe2x80x83                                ⁢                ν                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          1                )            
When the atom is in the laser beam, the rate of resonant scattering per unit time is given by                               sr          ⁡                      (                                          v                z                            ,              I                        )                          =                              Γ            ⁢                          xe2x80x83                        ⁢                          Ω              rabi              2                                                                          (                                  ν                  -                                      ν                    0                                    -                                                            v                      ⁢                                              xe2x80x83                                            ⁢                                              ν                        z                                                              c                                                  )                            2                        +                          Γ              2                        +                          Ω              rabi              2                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          2                )            
Here, xcex93 is the natural width of the transition. xcexa9Rabi is the Rabi frequency, which depends on the light intensity.
Using these relations, an atom with velocity vz attains an energy change per unit time xcex94Escatt due to the resonant scattering;                                           (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  E                  scatt                                                            Δ                ⁢                                  xe2x80x83                                ⁢                t                                      )                    ⁢                      (                                          v                z                            ,              I              ,                              Δ                ⁢                                  xe2x80x83                                ⁢                ν                                      )                          =                  h          ⁢                      xe2x80x83                    ⁢          Δ          ⁢                      xe2x80x83                    ⁢          v          ⁢                      xe2x80x83                    ⁢                      sign            ⁡                          (                              v                z                            )                                ⁢                      sr            ⁡                          (                                                v                  z                                ,                I                            )                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          3                )            
Here, sign (Vz) is a symbol that indicates the direction of the motion of the atom relative to the laser beam. It is negative when the directions of the atom and light beam are opposite, in which case the atom slows down. It is positive when the directions are the same, in which case the atom accelerates.
Next, the average energy and fluorescence intensity is calculated when laser-cooling is applied to the ions. Here, it is assumed that the ions are in a gaseous state (not in a Wigner-crystal state). The velocity distribution n (vx, vy, vz) can be written as a Maxwell distribution with an ion temperature T;                               n          ⁡                      (                                          v                x                            ,                              v                y                            ,                              v                z                                      )                          =                                            N              ⁡                              (                                  m                                      2                    ⁢                    π                    ⁢                                          xe2x80x83                                        ⁢                    kT                                                  )                                                    3              2                                ⁢                      exp            ⁡                          (                                                -                  m                                ⁢                                  xe2x80x83                                ⁢                                                                            v                      x                      2                                        +                                          v                      y                      2                                        +                                          v                      z                      2                                                                            2                    ⁢                    kT                                                              )                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          4                )            
Modifying this equation to one-dimension, the laser cooling efficiency, which is defined as the average energy change of an ion by laser-cooling per unit time, can be described as                                           (                          m                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                kT                                      )                                1            2                          ⁢                  ∫                                    (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                                      E                    scatt                                                                    Δ                  ⁢                                      xe2x80x83                                    ⁢                  t                                            )                        ⁢                          (                                                v                  z                                ,                I                ,                                  Δ                  ⁢                                      xe2x80x83                                    ⁢                  ν                                            )                        ⁢                          exp              ⁡                              (                                                      -                    m                                    ⁢                                      xe2x80x83                                    ⁢                                                            v                      z                      2                                                              2                      ⁢                      kT                                                                      )                                      ⁢                          ⅆ                              v                z                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          5                )            
The fluorescence intensity per unit time of an ion with temperature T, averaged over the velocity distribution, is                                           (                          m                              2                ⁢                π                ⁢                                  xe2x80x83                                ⁢                kT                                      )                                1            2                          ⁢                  ∫                                    sr              ⁡                              (                                  I                  ,                                      v                    z                                                  )                                      ⁢                          exp              ⁡                              (                                                      -                    m                                    ⁢                                      xe2x80x83                                    ⁢                                                            v                      z                      2                                                              2                      ⁢                      kT                                                                      )                                      ⁢                          ⅆ                              v                z                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          6                )            
In laser-cooled fluorescence mass spectrometry, an alternating-current electrical field is applied to excite the secular motion of the sample ions. The maximum heating rate by this AC electric field, which is defined as the maximum value of the energy increase xcex94Eheat per unit time, can be described as                                           (                                          Δ                ⁢                                  xe2x80x83                                ⁢                                  E                  heat                                                            Δ                ⁢                                  xe2x80x83                                ⁢                t                                      )                    max                =                                            (                                                2                  ⁢                  kT                                                  π                  ⁢                                      xe2x80x83                                    ⁢                  m                                            )                                      1              2                                ⁢                                    eV              ac                                      8              ⁢                              r                0                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          7                )            
where T is the ion temperature, Vac is the voltage amplitude of dipole AC electrical field applied to the electrodes. Hereafter, Equation 7 is derived.
A precise calculation of the actual heating rate (or the change of temperature) of the ions might not be simple because the Coulomb interaction between ions constitutes a many-body non-linear system, and because a precise calculation must include laser-cooling effect at the same time, which also depends non-linearly on the ion velocity. On the other hand, to determine a suitable set of operation parameters for the laser-cooled fluorescence mass spectroscopy, an approximate knowledge of the maximum heating rate in the absence of laser-cooling is quite useful as will be discussed later. Thus, we attempt to find the maximum upper limit for the heating rate under the following conditions:
(a) there is no laser cooling,
(b) applied AC oscillation frequency xcfx89 matches the secular motion oscillation xcfx890, and
(c) Coulomb interaction between ions is ignored.
That is, heating due to the trapping radio-frequency is ignored, since it is much smaller than the resonant forced-oscillation heating.
Further, we assume the equation of motion as a one-dimensional forced oscillation without damping:                                                         ⅆ              2                        ⁢            x                                ⅆ                          t              2                                      =                                            f              m                        ⁢                          exp              ⁡                              (                                  i                  ⁢                                      xe2x80x83                                    ⁢                  ω                  ⁢                                      xe2x80x83                                    ⁢                  t                                )                                              -                                    ω              0              2                        ⁢            x                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          8                )            
where f is the amplitude of the force. In the present case, AC electric voltage Vac is applied to two pairs of adjacent quadrupole electrodes, resulting in an approximate oscillating electric field amplitude f=eVac/(2r0).
When Equation 8 is solved for velocity v with initial conditions v(t=0)=0 and x(t=0)=0,                     v        =                                            ft                              2                ⁢                m                                      ⁢                          cos              ⁡                              (                                                      ω                    0                                    ⁢                  t                                )                                              =                                    v              0                        ⁢                          cos              ⁡                              (                                                      ω                    0                                    ⁢                  t                                )                                                                        (                  Equation          ⁢                      xe2x80x83                    ⁢          9                )            
where v0=ft/2 m.
The energy E at time t is given by                     E        =                                            1              4                        ⁢                          mv              0              2                                =                                                    f                2                                            16                ⁢                m                                      ⁢                          t              2                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          10                )            
Extending this result, we approximate the heating rate of any ion at velocity V0 by                                           ⅆ            E                                ⅆ            t                          =                                                            f                2                                            8                ⁢                m                                      ⁢            t                    =                                    f              4                        ⁢                          v              0                                                          (                  Equation          ⁢                      xe2x80x83                    ⁢          11                )            
Assuming a Maxwellian distribution of ion velocity V0 with temperature T, the maximum heating rate is approximated by                                                                                           Δ                  ⁢                                      xe2x80x83                                    ⁢                                      E                    heat                                                                    Δ                  ⁢                                      xe2x80x83                                    ⁢                                      t                    max                                                              =                                                                    (                                          m                                              2                        ⁢                        π                        ⁢                                                  xe2x80x83                                                ⁢                        kT                                                              )                                                        1                    2                                                  ⁢                                  ∫                                                                                    ⅆ                        E                                                                    ⅆ                        t                                                              ⁢                                          exp                      ⁡                                              (                                                                              -                                                          mv                              0                              2                                                                                                            2                            ⁢                            kT                                                                          )                                                              ⁢                                          ⅆ                                              v                        0                                                                                                                                                                    =                                                                    (                                                                  2                        ⁢                        kT                                                                    π                        ⁢                                                  xe2x80x83                                                ⁢                        m                                                              )                                                        1                    2                                                  ⁢                                                      eV                    ac                                                        8                    ⁢                                          r                      0                                                                                                                              (                  Equation          ⁢                      xe2x80x83                    ⁢          12                )            
FIG. 4 through FIG. 8 show calculated results of the temperature dependence of the laser cooling rate (or, cooling efficiency) of a typical laser-cooled ion 24Mg+ at various laser beam parameters. Each figure includes curves for various values of the detuning frequency. The horizontal axis shows the ion temperature. The vertical axis shows the laser-cooling rate. The laser-cooling rate has a maximum cooling rate at temperatures between about 1 and 100 K.
The figures also show the temperature dependence of the maximum heating rate at various values of analysis voltage Vac. The vertical axis shows the maximum heating rate. The laser beam, which is focused to a diameter of 0.2 mm, has a power of 1 xcexcW, 10 xcexcW, 100 xcexcW, 1 mW and 10 mW, respectively in each figure.
FIG. 9 through FIG. 13 show calculations of the temperature dependence of the fluorescence intensity of 24Mg+ ions at various laser beam parameters. Each figure includes curves for various values of the detuning frequency. The horizontal axis shows the ion temperature. The vertical axis shows the fluorescence intensity. The figures respectively shows results for laser beams of 1 xcexcW, 10 xcexcW, 100 xcexcW, 1 mW and 10 mW focused to a diameter of 0.2 mm.
FIG. 14 through FIG. 17 show calculated results of the temperature dependence of the laser cooling rate of a typical laser-cooled ion 138Ba+ at various laser beam parameters. The figures also show the temperature dependence of the maximum heating rate at various values of analysis voltage Vac. FIG. 18 through FIG. 21 show calculations of the temperature dependence of the fluorescence intensity of 138Ba+ ions at various laser beam parameters.
The following data are used in the calculations.
Mass of 24Mg+ - - - m=24 mu 
Resonance wavelength of 24Mg+ - - - xcex0=280 nm
Natural width of 24Mg+ - - - xcex93=43 MHz
Mass of 138Ba+ - - - m=138 mu 
Resonant wavelength of 138Ba+ - - - xe2x88x92xcex0=493 nm
Natural width of 138Ba+ - - - xcex93=15.1 MHz
In the calculations of 138Ba+, it is treated as a 2-level atom, where the pump-back transition is ignored, and only the laser-cooling transition of 138Ba+ is taken into account.
The above calculations give insight on the mechanism of how the mass-signal is produced in laser-cooled fluorescence mass spectrometry, and teaches us a valuable guidance on how to stably obtain the signal.
Firstly, the mechanism of the signal generation is considered from the relation between ion temperature and fluorescence intensity.
We consider the case of xcexa9Rabixe2x89xa6xcex93, where the resonance scattering is not strongly saturated. At detuning frequency smaller than natural width of the transition, the fluorescence intensity increases when the ions are laser-cooled and drop its temperature. (See for instance, the characteristic for a detuning frequency from 0 to 40 MHz in FIG. 9.). We observe this typical effect when laser cooling experiments are performed. When the detuning frequency becomes much larger than the natural linewidth, the fluorescence intensity reaches an maximum value at temperature around one to ten Kelvin (See for instance the characteristic in FIG. 9 when the detuning frequency is larger than xe2x88x9250 MHz.) This maximum occurs because the probability of absorbing laser light becomes larger with a wider velocity distribution due to the Doppler effect.
Next, we explain the case of xcexa9Rabi greater than  greater than xcex93, where the resonance scattering is strongly saturated. Though a strong fluorescence intensity could be obtained by causing saturation, the dependence of fluorescence intensity on ion temperature and detuning frequency became smaller (See for instance, the characteristic of FIG. 12 and FIG. 13.). This effect appears because the width of resonant absorption spectra widens due to saturation, so that the resonance scattering rate does not depend so much on the ion velocity at small detuning frequencies.
In laser-cooled fluorescence mass spectrometry, information on changes of temperature is obtained from the changes in fluorescence intensity. For maximum signal, it is necessary to maximize the change of fluorescence. Above arguments teach us that, to this end, it is desirable to keep the laser power low enough not to saturate the transition, and to keep the detuning to zero.
Next, we discuss the conditions for maintaining non-destructive analysis using the relation between ion temperature and laser cooling rate.
In our calculation, the laser-cooling rate has a maximum cooling rate at temperatures between about 0.1 K and 100 K when xcexa9Rabi less than xcex93. At lower temperature, the cooling rate decreases as the temperature decreases. The laser cooling rate approaches zero as the temperature decreases, because, in our approximation, the decrease of the width of the Doppler velocity distribution results in the decrease of mean energy loss due to photon absorption. In reality, as the temperature approaches zero, the heating effect from natural emission of photons by excited ions must be taken into account, whose balance with the cooling effect by photon absorption will determine the lowest temperature attainable. Since the temperature in our calculation is much higher than the temperature where such a heating by natural emission becomes important, we considered only the cooling effect by absorption.
Above the maximum cooling rate, the rate drops as the temperature rises. At higher temperatures, the laser cooling rate decreases due to decreased probability of photon absorption.
We now describe ion stability in laser-cooled fluorescence mass spectrometry, using these calculation of the laser cooling rate and the maximum heating rate by the forced oscillation. Ion loss may occur when the maximum heating rate is larger than the laser cooling rate.
When the sample ions are oscillated by external fields, the heating rate is proportional to the number of sample ions, and the cooling rate is proportional to the number of laser-cooled ions. The heat coupling between the laser-cooled ions and the sample ions depends on the sympathetic cooling rate. In the following typical calculation, the number of ions is set equal to the number of laser-cooled ions, and the calculation assumes that the sympathetic coupling is complete, i.e., that there is no temperature difference between the sample ions and the laser-cooled ions.
When the maximum heating rate from heating (hereafter, analysis heating) due to the forced oscillation of the ion by the analysis voltage is smaller than the laser cooling rate, ions are not lost due to analysis, and are stable. At a fixed set of values of the analysis-voltage amplitude and laser beam parameters, the intersection of the maximum-heating-rate line and the laser-cooling curve provides a guide for establishing the temperature where the cooling and heating are balanced. Of the two intersections that may exist, the intersection on the lower temperature side provides a guide for the ion temperature during analysis heating. If the heating rate due to analysis heating at this ion temperature is increased by increasing the analysis voltage, the ion temperature shifts to a higher value. When the analysis voltage is further increased, the maximum heating rate line and the laser cooling line come in contact at a single point. See, for instance, the contact made by the line for analysis voltage 0.9 mV in FIG. 4 and the line for the detuning frequency xe2x88x9230 MHz. This point yields the maximum temperature that can be stably reached under that specific laser cooling conditions. When a larger analysis voltage is applied to provide further heating, the analysis heating rate exceeds the laser cooling rate, so that the ions are heated at any temperature without reaching a stable balance with cooling, and ions are lost. That is, an upper limit for analysis voltage exists at a fixed laser-cooling condition where analysis is performed without losing ions. Hereafter, this temperature is referred to as the maximum ion temperature, and is shown by the white circles in the FIGS. 4-8, FIGS. 14-17, FIG. 22, and FIG. 23.
Therefore, in laser-cooled fluorescence mass spectrometry as shown above, by applying cooling that is stronger than the applied forced-oscillation, the laser-cooled ions and sample ions can be trapped stably in the ion trap. Preferably, parameters should be chosen so that strong and stable cooling is realized over as wide an ion temperature range as possible in the presence of temperature increase due to analysis heating. The present calculations show that, for effective and stable cooling, the intensity of the laser beam should preferably be so strong as to saturate the transition, and the laser detuning should preferably be much larger than the natural linewidth of the transition. The extent of saturation of the intensity should not be so strong as to lowering the laser-cooling efficiency itself.
We point out two issues in the laser-cooled fluorescence mass spectrometry of the prior art. One issue is that finding the laser cooling conditions for obtaining a signal is difficult in general. For instance, when the number of ions changes, then new conditions must be searched for. Another issue is that even if conditions are found for obtaining a signal, loss of ions in the trap frequently occurs due to analysis heating.
As shown before, in-situ (in-trap) analysis can be performed in principle in the spectrometry of the prior art. However, to make in-trap analysis possible, a plurality of strict optimal parameters are required. If optimal conditions are not provided, then the ions are lost, or no signal can be obtained. Since selecting the correct analysis parameters is difficult, considerable experience is needed to implement the spectrometry of the prior art.
The intent of this invention is to overcome these two issues of the prior art. In the prior art, these two issues arise because one species of ions is simultaneously used both as a laser-cooled coolant means and as an ion temperature probe means. In the prior art, a large detuning frequency (preferably much larger than the natural linewidth) is required to obtain sufficient laser-cooling efficiency over a wide temperature range. On the other hand, small detuning frequency (preferably smaller than the natural linewidth) is required to obtain a strong signal intensity (change of fluorescence intensity). These two conflicting conditions are the cause of the issues of the prior art. This present invention resolves these mutually conflicting conditions, by providing independent and isolated means for the laser-cooling coolant and the ion-temperature probe.
Hereafter, a detailed description of the invention is given. First of all, a method for effectively selecting the laser cooling conditions is explained.
In order to increase the change of the fluorescence intensity of the ion-temperature probe after analysis heating, ion temperature before analysis should preferably be set to a low temperature below 1 K. However, if the temperature is too low (below 0.1 K), ion crystallization may occur, so that the oscillation frequency may differ from the secular motion frequency, which complicates data analysis. To simplify the analysis, a gaseous phase of ion should be preferably achieved by a balance between the laser cooling and heating, for which heating, typically, a trapping-radio-frequency heating effect is dominant in the absence of analysis heating. A large detuning frequency of xe2x88x92100 MHz or more, which is much larger than the natural linewidth of the transitions presently used, is utilized for the laser-cooling beam to achieve a gaseous phase within temperature 0.5 K to 1 K. The laser intensity is set to a saturated intensity with saturation parameter xcexa9Rabi/xcex93≈1 to 5. For effective probing, the laser beam for the probe light should preferably have a detuning frequency much smaller than the natural linewidth, preferably in the vicinity of 0 MHz (xcex94xcexd=xe2x88x9220 MHz to 20 MHz: optimized at 0 MHz), and its intensity should preferably set below the saturation intensity (saturation parameter: from approximately xcexa9Rabi/xcex93=0.1 to 1).
Two methods for separating the laser cooling means and the ion temperature probe means are explained next.
Method (1): A method with separate ion species for the laser cooled ions and the probe ions.
In this method, separation of the laser cooled means and the ion probe means are realized by using two ion species each supporting their respective function. To obtain strong laser-cooling, one ion species, which can be laser-cooled, is used as the laser-cooled ion using a saturating laser light with a large detuning frequency. To generate probe fluorescence, another ion species, which is able to be laser-cooled, is used as the probe ion, by utilizing a weak laser light in the vicinity of the 0 MHz detuning frequency. Effective operation can be attained within the natural width, typically at approximately xcex94xcexd=xe2x88x9220 MHz to 20 MHz; however, 0 MHz is optimal, so that one can monitor the changes in fluorescence due to analysis heating of the sample ions.
It is effective to use two different isotopes of the same element as the two species of atomic ions for the laser cooled ions and the probe ions. In such a case, the two fluorescence wavelengths will be nearly identical. By using a method such as adding the laser intensity modulation to the probe light and then extracting the fluorescence intensity components synchronized with that modulation, only the fluorescence emitted by the probe ions can be monitored. For instance, by generating probe light intermittently with an optical chopper, the fluorescence intensity emitted by the probe ions will be detected only when the probe light is On.
Method (2): A method using a single ion species and two laser beams, one for a laser cooling means and the other for a probing means.
The separation of the laser cooling means and the probing means can be realized by using at least two laser beams to excite single species of laser-coolable ions to be used both as a coolant and a probe. One laser beam is used as a laser cooling means providing a saturating laser light at a large detuning frequency. Another laser light is used as a probe means providing a weak laser light substantially at a detuning frequency of zero (0). Fluorescence generated by the probe light is monitored.
The laser-cooling beam saturates the cooling transition of the ions. This saturation will affect the fluorescence excited by the probe beam. This reduces the ability of the probe beam to detect the ion temperature. Following two methods are effective in avoiding this deterioration in the ability to monitor ion temperature. In one method, the energy level used in laser cooling and the energy level used in the probe are separated. Separating these energy levels means that effects on probe light saturation can be avoided. In the other method, the laser-cooling light and the probe light excite the same level, but the laser-cooling light is stopped during observation of the fluorescence from the probe light. One example of achieving this intensity modulation is to use an optical chopper on the laser-cooling beam, so that one detects the fluorescence by the probe light when the cooling beam is off.